LNB downconverters are widely used with direct broadcast satellite (DBS) television receivers for converting microwave signals to an intermediate frequency suitable for processing by an indoor tuner. The LNB downconverter is typically mounted on an antenna or satellite dish and enclosed in a weatherproof housing. The output is connected to an indoor tuner attached to a television or video cassette recorder (VCR).
The LNB downconverter typically includes a frequency downconverter mounted on a printed circuit board along with support circuitry, additional amplifier stages and filters to provide increased amplification and reduced front end noise. The frequency downconverter typically converts microwave frequencies in the range of 11 to 12 GHz to an intermediate frequency (IF) in the range of 1,000 to 2,000 MHz.
With the rapid development of GaAs MMIC technology, commercial GaAs MMIC frequency downconverters have become very feasible for LNB applications. This is because GaAs MMICs provide low cost, improved reliability, reproducibility, small size, low weight, broadband performance, circuit design flexibility and multifunctional performance on a single chip. Today, several companies are offering competitive devices built from GaAs technology to participate in the development of the large consumer market for direct broadcast satellite receivers. However, to remain competitive, manufacturers must come up with ways to reduce cost. Reduced costs can be achieved by the elimination of circuitry and the consequent reduction of the dice area.
One technique for reducing the input circuitry of the GaAs MMIC frequency downconverters, and thereby reduce the dice area, is to use gate pumped active field effect transistor (FET) mixers. The principal advantages of the gate pumped FET mixer over other types of active FET mixers is that it can provide lower noise figures and higher conversion gain. Based on the low noise performance of this device, the gate pumped FET mixer in conjunction with an IF amplifier chain can achieve comparable gain and noise figure without the RF pre-amplifiers and image rejection filters typically integrated on GaAs MMIC frequency downconverters.
Despite the above mentioned advantages of the gate pumped mixer, a significant drawback is that this topology offers no isolation between the microwave and local oscillator (LO) signal ports. One way to overcome this problem is to design a single balanced gate pumped mixer and separate the microwave signal from the LO signal with a 180.degree. rat-race hybrid coupler. By using a hybrid coupler, 30 dB of isolation at the center frequency is typically achieved. The disadvantage of this approach, however, is that FET balanced mixers require a 180.degree. output hybrid, such as a passive IF balun, which complicates the circuitry and may increase its size.
Accordingly, there is a current need for a GaAs MMIC single balanced frequency downconverter that does not require an output hybrid.